1. Field of the Invention
The present invention relates to an electric current detecting apparatus using a magnetoelectric transducer and more particularly to an electric current detecting apparatus of a magnetic balanced type using a magnetoelectric transducer such as a Hall effect element.
2. Description of the Prior Art
An operation principle of an electric current detecting apparatus of a magnetic balanced type using a magnetoelectric transducer such as a Hall effect element will be explained with reference to FIG. 1. In FIG. 1, a magnetic flux is generated in a looped core 2 by a primary current I.sub.1 flowing through a primary winding or conductor 1. A magnetic flux density in a gap 3 is measured by a magnetoelectric transducer 4 such as a Hall effect element. An output from the transducer 4 is applied to an amplifier 6 which functions as a current supply source to a secondary winding 5. That is, a secondary current I.sub.2 is supplied from a current supply source 6 to the secondary winding 5 which is wound around the looped core 2 so that the magnetic flux density in the core 2 is cancelled to create a magnetically balanced condition. Under the magnetically balanced condition, the primary current I.sub.1 can be measured by measuring the secondary current I.sub.2 by an ammeter 12 arranged in the path between the amplifier 6 and the secondary winding 5, based on an equi ampere turns law where a magnitude of a magnetomotive force on a primary side is equal to that of a secondary side in a magnetically balanced condition.
Since either a dc current or an ac current can be measured by this electric current detecting apparatus of a magnetic balance type in a non-contact state and with a high accuracy, it has been frequently used in such a field as an inverter control system. However, a conventional apparatus has a disadvantage that high frequency characteristics are poor. This means that a measurement accuracy at a high frequency is deteriorated.
Especially in a recent inverter control system, a switching speed is required to be increased, so that an electric current detecting apparatus of a dc/ac type having good high-frequency characteristics is required. For instance, when a square wave is measured in pulse width modulation (PWM) control, a response delay of the secondary current I.sub.2 occurs as shown in FIG. 2, and an electric current measurement accuracy is deteriorated because of waveform distortion or a peak value error. This is caused by deteriorated high-frequency characteristics of an electric current detecting apparatus, resulting in that high harmonic components included in the square wave of the primary current I.sub.1 disappear from the secondary current I.sub.2.
The present inventors examined the above response delay and have found that the response is delayed due to an increased leakage magnetic flux caused by an increased magnetic reluctance by the gap 3 of the looped core 2. Based on the above fact, the inventors invented the present invention.
Conventionally, various kinds of proposals on improvement of an electric current detecting apparatus have been offered. These proposals, however, have not yet developed an electric current detecting apparatus with sufficient performance.
For example, FIGS. 3 to 5 show an electric current detecting apparatus in which the magnetoelectric transducers 4 are disposed correspondingly in the gaps 3 arranged distributedly in the looped core 2. The secondary windings 5 accommodated in bobbins 9 are so arranged to surround these gaps 3, respectively. This electric current detecting apparatus can improve a measurement accuracy in the case of a dc or low frequency ac current, and particularly can avoid an error resulting from a change in a shape or a location (for instance, a center or a corner inside of the looped core 2 or the like) of the primary winding. However, this electric current detecting apparatus is not effective to improve high frequency characteristics.
In an electric current detecting apparatus as shown in FIGS. 6 to 8 where, the magnetoelectric transducer 4 is embedded in the gap 3 arranged in one portion of the core 2, in an electric current detecting apparatus which employs magnetic coupling between the primary winding and a core without a gap, or in an electric current detecting apparatus as shown in FIG. 9, where a yoke or a shield 13 is provided inside the secondary winding 5 in the bobbin 9, a response delay is improved. However, when the primary current changes greatly and rapidly, a damping oscillation (ringing) occurs, so that a settling time is increased and accordingly waveform distortion or a peak value error is unexpectedly deteriorated.